1. Field of the Invention
The present invention relates to a musical tone synthesizing apparatus which synthesizes musical tones by effecting delay processes on excitation signals applied thereto.
2. Prior Art
FIG. 12 is a block diagram showing a simplified configuration of an example of the musical tone synthesizing apparatus conventionally known. In FIG. 12, a numeral 1 denotes an excitation-signal generating circuit which generates and continuously outputs excitation signals, wherein each of those excitation signals contains a plenty of noise components but does not contain a component for a tone-pitch parameter (hereinafter, simply referred to as a tone-pitch component). Numerals 2 and 3 denote loop circuits, both of which have the same configuration at least containing a delay circuit and a filter. A loop delay which is occurred when the signal circulates through each of the loop circuits 2 and 3 at one time is set identical to a tone-pitch period of the musical tone to be produced. A comb-like frequency characteristic is employed by each of the loop circuits 2 and 3. Herein, the lowest frequency, which corresponds to one of the peaks of the comb-like frequency characteristic but which is not equal to 0 Hz, coincides with the tone-pitch frequency of the musical tone to be produced, while each of the frequencies, which correspond to the other peaks of the comb-like frequency characteristic, is roughly equal to a multiple of the tone-pitch frequency of the musical tone to be produced. Hence, by supplying the signal to the loop circuits 2 and 3, the tone-pitch component will be incorporated into the signal.
Next, the description will be given with respect to the reason why the loop circuits 2 and 3 are connected in series as shown in FIG. 12. When using one of the loop circuits 2 and 3 only in the musical tone synthesizing apparatus, in order to increase an amount of tone-pitch components, the comb-like frequency characteristic which is applied to the signal circulating through the loop circuit should be modified such that the peaks of the comb-like frequency characteristic are made sharper. In order to do so, it Is necessary to increase the loop gain of the loop circuit.
However, If the loop gain of the loop circuit is increased excessively, a transient-response period should become longer, which raises a problem that the musical tone cannot be attenuated soon and the stability in the operation of the loop circuit may be somewhat damaged. In the worst case, an operational error should be happened. For this reason, the comb-like frequency characteristic employed in the loop circuit is set such that the peaks are not made sharper so much. However, this causes another problem that the tone-pitch component cannot be made clear.
Thus, the two loop circuits 2 and 3 are connected in series in the circuit shown in FIG. 12. This circuit can offer a sharper peak for the comb-like frequency characteristic, embodied by the loop circuits 2 and 3 as a whole without making the transient-response period longer so much. By continuously applying the excitation signal, which does not contain the tone-pitch component, to those loop circuits 2 and 3, it is possible to synthesize a sustain-type musical tone having a high quality in its sustain portion.
Meanwhile, some of the musical tone synthesizing apparatuses conventionally known are designed to use a single loop circuit which can offer a sufficient amount of tone-pitch components. Another example of the musical tone synthesizing apparatus is designed as shown in FIG. 13, wherein the loop circuits 2 and 3 are connected in parallel and their outputs are added together by an adder 4. Particularly, when synthesizing a decay-type musical tone, the loop gain of each loop circuit should be increased, while the excitation-signal generating circuit 1 should generate the excitation signal instantaneously. Such excitation signal instantaneously generated is supplied to the above-mentioned single loop circuit or the loop circuits connected in parallel. Thus, by using the transient-response period of each loop circuit which is relatively long, the decay-type musical tone can be synthesized.
In the circuit shown in FIG. 12, an output signal of the loop circuit 3 represents the synthesized musical tone containing the tone-pitch component. In the circuit shown in FIG. 13, an output signal of the adder 4, which adds the outputs of the loop circuits 2 and 3 together, represents the synthesized musical tone containing the tone-pitch component.
In the musical tone synthesizing apparatuses conventionally known, it is possible to synthesize the musical tones containing the tone-pitch components as described above. However, in the synthesized musical tone, overtone components should be artificially incorporated and they are disposed orderly in the frequency characteristic of the synthesized musical tone. In other words, there is a drawback that as compared to acoustic musical tones, which are produced from acoustic musical instruments, the synthesized musical tones are unsatisfactory for the listeners.
Now, by taking an example of the violin, the complexity of the acoustic musical tone produced by the acoustic musical instrument will be described in detail. In the violin, by bowing a string (or strings), the musical tone is produced. In this case, energy, which is produced when the performer operates the bow, is transmitted to the string so that vibration is occurred and is transmitted through the string between its both-side terminals In a manner of reciprocating motion. That vibration causes waves of the string, so that the body of violin resonates to those waves. Hence, the waves are acoustically amplified, so that corresponding sound waves are radiated into the air.
In general, the violin does not merely produce the sounds corresponding to the waves of vibration which are transmitted between the both-side terminals of the string in a reciprocating manner. Other than those sounds, the violin can produce frictional sounds as well, which are produced due to the friction between the bow and string. Those frictional sounds are radiated into the air from a point at which the bow comes in touch with the string or from the bow. Those sounds do not occupy the main part of the violin sounds, however, a part of them is certainly transmitted to the ears of the listener. Those frictional sounds contributes to a unique acoustic effect of the violin sounds. In other words, the real acoustic characteristic which is obtained when a part of the frictional sounds is radiated into the air is different from the acoustic characteristic which is obtained when the waves, which are produced when the body of violin resonates to the waves transmitted through the string only, are acoustically amplified and are radiated into the air.
As described above, the acoustic sound produced from the acoustic musical instrument contains the tone-pitch component and the complexity, so that the acoustic sound can offer a unique acoustic effect to the listener as compared to the musical tone artificially synthesized.
In the musical tone synthesizing apparatuses described before, the loop circuits 2 and 3 are designed to simulate the reciprocating transmission of the waves which are transmitted between the both-side terminals of the string of the violin. The output signal of the loop circuit 3 in the circuit shown in FIG. 12 or the output signal of the adder 4 in the circuit shown in FIG. 13 is amplified by the amplifier (not shown); and then, the output signal is converted into the sound by a so-called electric sound converter such as the speaker (not shown). The above amplification and electric conversion may correspond to the aforementioned phenomenon in which the body of violin resonates to the waves reciprocating between the both-side terminals of the string so that those waves are acoustically amplified; and then, the corresponding sound waves are radiated into the air.
In the acoustic musical instrument such as the violin, the musical tone acoustically produced contains the noise component such as the frictional sound, which is produced due to the friction between the bow and string, as well as the main component corresponding to the sound produced from the waves of vibration on the string. However, the musical tone synthesizing apparatuses conventionally known are not designed to synthesize the noise-component sounds other than the main-component sounds. In short, the musical tones artificially synthesized are unsatisfactory for the listener as compared to the acoustic sounds.
In the musical tone synthesizing apparatus, each of circuit elements is normally embodied by the digital circuit. In the circuit shown In FIG. 13, several kinds of parameters such as the tone-color parameter are used for each of the loop circuits, whereas the output signals of the loop circuits are added together by the adder. However, every time the parameter used for the loop circuit is changed, the level of the output signal of the loop circuit should be changed.
In some cases, the value of the output signal of the adder or the output value of each circuit element may exceed a limit value which corresponds to the predetermined number of bits employed by the musical tone synthesizing apparatus. In short, an overflow event is occurred. If such overflow event is occurred, the musical-tone waveform corresponding to the output signal of the adder should be somewhat distorted so that the desired musical tone cannot be obtained. Such drawback can be overcome by increasing the number of bits used for each of the circuit elements such as the adder to the satisfactory number. However, this results In an increase of the size of the circuit element, which will lead to a raise of the cost required for manufacturing the musical tone synthesizing apparatus.